The present invention relates generally to pressure regulators and controllers and, more particularly, to pressure regulator systems for metal forming processes, particularly titanium superplastic forming.
In a process known as superplastic forming of titanium, a titanium article is formed by heating titanium sheet stock to a plastic state in a furnace and pressing it against a mold, or die, under force of a pressurized inert gas, usually argon. In an improved version of this process, the argon pressure is varied with time over a critically defined pressure profile such that the titanium sheet stock undergoes deformation at a relatively slow and substantially constant strain rate throughout the forming process. The actual pressure profile as a function of time is determined in each case by the shape of the die and the article to be formed thereof, the thickness of the titanium stock, and other factors. The requisite pressure profile differs substantially from one article to another, and each profile is typically a relatively complex, nonlinear function. The absolute gas pressures required range from 25 to 500 psi or more, and must be maintained for periods of a few seconds to a few hours. There has not been available previously a pressure regulator suitable for varying the argon pressure in the forming chamber in accordance with such predetermined pressure profiles.
Certain aspects of the improved titanium forming method render regulation of the argon pressure by conventional devices and methods impractical. First, the argon pressure must be rigorously maintained within a narrow pressure window defined by upper and lower pressure limits, which limits vary continuously in a nonlinear manner over time. As mentioned above, the actual profile over which the limits vary depends in each case on the shape of the forming die and the thickness of the titanium sheet stock. Secondly, the volume of argon gas within the pressure chamber is often quite small, on the order of two to three cubic inches, such that conventional pressure regulator devices are typically incapable of providing the precise control necessary to maintain the pressure in the chamber within the desired pressure limits. Further, the progressive deformation of the titanium stock continuously changes the volume of the argon gas in the forming chamber, such that the pressure regulator must be capable of controlling the pressure in accordance with the predetermined profile even as the volume inside the forming chamber is changing. Finally, the temperature inside the chamber is also varied over the course of a forming cycle, another variable that tends to vary the chamber pressure and which must be compensated for by the pressure regulator.
A typical pressurization cycle may last up to several hours. The success of the cycle is dependent on strict regulation of the argon pressure in accordance with the predetermined profile. Since even relatively transient excursions from the profile may represent a failure in the system, it is desirable to provide a warning indicator to indicate the occurrence of an excursion of the chamber pressure outside predetermined pressure limits for an extended period of time, for example, 30 seconds. Such a prolonged excursion is ordinarily indicative of a failure condition, such as bursting of the titanium stock or a leak in the apparatus. In the event of such a failure, even if only transient, it is usually desirable to terminate the run and start over.
Accordingly, it is the object and purpose of the present invention to provide a pressure regulator for a metal forming chamber, particularly a titanium superplastic forming chamber.
It is also an object and purpose of the present invention to provide a pressure regulator capable of regulating gas pressure over a programmable, predetermined pressure profile.
It is a further object and purpose of the present invention to provide a pressure regulator capable of regulating argon pressure in a titanium superplastic forming chamber, particularly a chamber of relatively small volume and maintained at a high temperature.
It is a further object to provide a pressure regulator attaining the foregoing objects and purposes and which includes a warning indicator to provide an indication of pressure excursions outside predetermined high and low pressure limits for greater than a predetermined period of time.
It is yet another object to provide a pressure regulator that obtains the foregoing objects and purposes and which also enables the pressure in the forming chamber to be rapidly reduced by opening of a dump valve either manually or under program control from the regulator.